Heretofore, a number of retroreflective sheeting products have been marketed. A typical example is characterized by a single layer of tiny transparent microspheres embedded in a thermoset polymeric bead bond layer which is backed by a specularly reflective layer and an adhesive with a strippable protective layer, and surfaced with a transparent thermoset topcoat. Such sheeting is known as "embedded-lens" sheeting and was first taught by Palmquist et al., U.S. Pat. No. 2,407,680. Such sheeting has been sold commercially for many years in large volume and to the general satisfaction of its users. Despite this general satisfaction, there has been a desire for an improvement in certain properties of the sheeting. For example, it is desired to produce retroreflective sheeting which has greater durability and greater ability to withstand weathering than retroreflective sheeting heretofore known in the art. It is further desired to provide retroreflective sheeting which has improved reflectivity.
The retroreflective sheeting of the present invention, has improved physical and chemical properties over the retroreflective sheeting of the prior art. The improved retroreflective properties of the sheeting are believed to be due to the particular polymeric bead bond layer which in its uncured state may be softened by the application of heat and has a relatively nontacky character in the softened condition. This bead bond composition is thermally set after the application of glass beads to form a substantially infusible and insoluble crosslinked bead bond layer comprising urethane and/or urea linkages.
The retroreflective sheeting of the present invention is capable of carrying a greater concentration of transparent microspheres than most of the prior art sheeting, and consequently has greater retroreflectivity than most prior art sheetings. The higher concentration of microspheres is due to the nontacky thermoplastic character of the uncured softened polymeric bead bond layer, which allows for repositioning of the microspheres so that more beads may be forced into a unit area.
The bead bond materials generally known in the art are tacky in their soft or cured state. When exposed to a mass of glass beads or microspheres, these materials have a tendency to pick up the beads in multiple layers rather than a monolayer and, because of their tacky nature, do not allow for repositioning of the beads to maximize the number of beads per unit area. For example, U.S. Pat. No. 3,795,435 describes a retro-reflective sheeting which employs a bead bond layer prepared from a two-part polyurethane. The patent states that this bead bond layer has an adhesive character. Thus, it would not be able to carry transparent microspheres in as high a concentration as the retroreflective sheeting of to the present invention.
Additionally, the thermally cured polymeric bead bond layers of this invention, particularly those utilizing low molecular weight crystalline polyols as crosslinking agents, possess superior physical properties including impact resistant, low temperature flexibility and high temperature stability, superior elongation properties, external durability, and resistance to yellowing. Exterior durability and resistance to yellowing upon exposure to exterior environments are essential to the performance of retroreflective sheeting, particularly when used in exterior applications. Crystalline polyols have heretofore not been useful in preparing cured polymers which contain urethane or urea linkages since they are substantially insoluble in aprotic solvents, which are necessary solvent systems for the manufacture of such polymers. However, the method of copending U.S. patent application, U.S. Ser. No. 333,810, overcomes the problem of the limited solubility of crystalline polyol crosslinkers in aprotic solvents and allows these crosslinkers to be used in the synthesis of cured bead bond layers comprising urethane and/or urea linkages.